<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <title>14.getSnapShotBeforeUpdate的使用场景</title>
    <style>
        .list {
            width: 200px;
            height: 150px;
            background-color: skyblue;
            overflow: auto;
        }

        .news {
            height: 30px;
        }
    </style>
</head>

<body>
    <!-- 准备好一个“容器” -->
    <div id="test"></div>

    <!-- 引入react核心库 -->
    <script type="text/javascript" src="../js/17.0.1/react.development.js"></script>
    <!-- 引入react-dom，用于支持react操作DOM -->
    <script type="text/javascript" src="../js/17.0.1/react-dom.development.js"></script>
    <!-- 引入babel，用于将jsx转为js -->
    <script type="text/javascript" src="../js/17.0.1/babel.min.js"></script>

    <script type="text/babel">
        class NewsList extends React.Component {

            state = { newsArr: [] }

            componentDidMount() {
                setInterval(() => {
                    //获取原状态
                    const { newsArr } = this.state
                    //模拟一条新闻
                    const news = '黄晓果' + (newsArr.length + 1)
                    //更新状态
                    this.setState({ newsArr: [news, ...newsArr] })
                }, 1000);
            }

            getSnapshotBeforeUpdate() {
                //新数据之前的内容区高度
                return this.refs.list.scrollHeight
            }

            componentDidUpdate(preProps, preState, height) {
                //停留在原来的位置
                this.refs.list.scrollTop += this.refs.list.scrollHeight - height
                
                //直接到顶部
                // this.refs.list.scrollTop = 0
            }

            render() {
                return (
                    <div className="list" ref="list">
                        {
                            this.state.newsArr.map((n, index) => {
                                return <div key={index} className="news">{n}</div>
                            })
                        }
                    </div>
                )
            }
        }
        ReactDOM.render(<NewsList />, document.getElementById('test'))
    </script>
</body>

</html>